With printing machines, the application in the register of images on the print substrate is of great importance. In particular with multicolor printing, in which the overall colored image includes individual single-colored images that are printed on top of one another, registerability is important. The state of the art discloses a number of documents on this subject.
In one solution, for this purpose, register marks are applied to a conveyor belt or a sheet that are detected with sensors. Automatic deviations were determined in this manner, which were, for example, corrected by servo motors. Each register mark is allocated to an individual single-colored image. By the appropriate arrangement of the register marks, displacements of the register marks in the moving direction of the conveyor belt, so-called intrack deviations, and across from them, so-called crosstrack deviations, are detected. The intrack and crosstrack deviations are non-linear, non-cyclical errors that occur at random. In addition, register errors occur for a reason that is described below. During the application of an image on the sheet, an image-carrying illustration drum or intermediate drum rolls off onto the sheet and transfers the image; and, on the opposite side, a nip roller is arranged, which provides a counteracting force to the pressing force of the illustration drum or intermediate drum. Consequently, the term “printing drum” is used for the terms “illustration drum” and “intermediate drum”. The printing drum rolls off the conveyor belt at this point, and is identical to the illustration drum or with the intermediate drum, depending on whether the printing machine uses an intermediate drum or the image is directly transferred from the illustration drum to the conveyor belt.
Ideally, a constant contact force of the nip roller from below the conveyor belt is provided. With a constant contact force, the speed of the printing drum arranged above the conveyor belt remains the same, which does not change its speed due to the changing contact forces with the nip roller. However, out-of-roundness of the nip roller causes contact forces of the of the nip roller on the conveyor belt to change. Consequently, the rubber material of the printing drum, which touches the conveyor belt on the top side, is deformed and its speed changes.
As is understandable, a speed change of the printing drum, illustration drum or intermediate drum carrying the image causes an incorrect application of the image and the register marks on the sheet. The concentricity tolerance of the illustration drum or of the intermediate drum results in an erroneous application of the image and the register marks on the sheet. However, the out-of roundness tolerance of the illustration drum or the intermediate drum causes changing contact forces of the nip roller on the conveyor belt. The higher the contact force of the nip roller, the slower the printing drum, as a result of which the application of the image is delayed. The out-of-roundness tolerances of the illustration drum or the intermediate drum and a nip roller gripping the conveyor belt from the underside can be reduced by manufacturing measures. The disadvantage of this possible solution results in higher costs.